1. Field of the Invention
This invention relates to a process for making heat insulating firebricks especially heat-insulating firebricks which are comparatively light in weight.
2. Description of the Prior Art
Heat-insulating firebricks are conventionally formed by a manufacturing method which includes forming a first mixture of a refractory clay and a highly refractory aggregate (e.g. alumina, alumina silica, silicic acid, magnesia or zirconia); adding a granular, combustible pore-forming agent (e.g. wood chips, foamed polystyrene or coke) to the first mixture to form a second mixture; adding water to the second mixture to provide a moldable mixture; kneading (mixing) the moldable mixture; forming the moldable mixture into a shaped product by extruding, pressing or casting; drying and firing the shaped product; and, if necessary, finishing the fired product.
In some manufacturing methods the pores in the firebricks are formed by the addition of naphthalene, by the addition of substances which will chemically react to form bubbles, by the direct addition (physical addition) of bubbles, or else the weight of the firebricks is controlled by the addition of a light refractory aggregate or natural diatomaceous earth. However, these manufacturing methods have disadvantages as well as advantages.
Heat-insulating firebricks must display a low thermal conductivity and have a low bulk specific gravity (with a given shape, a given pore size and distribution, a given sintering degree and a given chemical composition, the lower the bulk specific gravity the lower the thermal conductivity), they must have a high strength for their bulk specific gravity and they must be capable of withstanding the high temperatures of the environments in which they are to be used. In addition, they must display low shrinkage and low deformations during their manufacture, and the materials from which they are made must be easily shaped. Also, the combustibles, when used as the pore-forming agents, must be smoothly burning. And of course the manufacturing method must be highly productive.
However, since many of the foregoing requirements are contradictory, no completely satisfactory method for manufacturing firebricks has yet been devised.
In Japanese Patent Publication No. 15232/35, published Oct. 13, 1960, a firebrick-forming method is discussed wherein light-weight, heat-insulating firebricks having good properties are cast in molds, the moldable mixture including foamed polystyrene as the pore-forming agent. The foamed polystyrene will not absorb much water and thus reduced amounts of water will be required for the casting step, and because the foamed polystyrene is spherical, the firebrick products will be uniform in composition, low in shrinkage when fired and reheated and high in strength for their bulk specific gravity. Also foamed polystyrene, being low in calorific content, will burn in an easily controlled manner.
However, although foamed polystyrene absorbs only small amounts of water during casting, nevertheless these absorbed amounts are higher than in some other molding methods and the products formed will likely display a great shrinkage when dried. Unless gypsum molds are used (these molds absorbing water), the shaped products cannot be removed from the molds until dried. Thus the casting technique in this Japanese Patent Publication using foamed polystyrene is less efficient than when using press-molding or extrusion-molding. Also, when dehydrated, the foamed polystyrene will likely become trapezoidal, thus producing a firebrick product which will be unstable in shape. And even though the slip in the mold could be stabilized with gypsum, cement, size or resin, such a large amount of these stabilizing materials would need to be used that the chemical composition of the firebricks would vary greatly, thus interfering with the drying and firing steps and resulting in a long and incomplete hardening of the slip.
It is thus an object of the present invention to provide an improved process for manufacturing heat-insulating firebricks using a method wherein the mixture forming the firebricks is cast in a mold, the mixture including added agents for quickly setting the firebrick-forming slip in the mold, these agents otherwise having substantially no influence on the chemical composition of the firebricks.